Sharif Digital Repository

In triangulation (or angle of arrival) target localisation method, the surface of the Earth is approximated by a flat plate. This approximation, while acceptable for low-altitude observers (such as unmanned aerial vehicles), is not a correct model for high-altitude observers (such as low Earth orbit - LEO satellites). In this study, the authors have discussed triangulation method using a LEO satellite observer over a spherical surface and have shown that, using spherical trigonometry and singular value decomposition (SVD), a straight forward solution can be found for target localisation problem. Also, error bound is discussed and analysed using both SVD as well as simulation. © The... 

Recently small satellites are used more commonly because of the low launching cost and development of microelectronics. Also lower weight, size, cost and the power consumption of magnetorquer, has made the application of them in controlling attitude of the satellites common. Intensive changes and non-ability of geomagnetic field are some of the problems, which have limited the efficiency of magnetorquers. In this paper a new control method is presented that keeps the attitude of satellite in desired condition only by electromagnetic coils. The distinction of this method is its abilities in comparison with other methods. In this analytic method a direct relation between design parameters,... 

The estimation of the satellite orbital elements using the integrated magnetometer and horizon sensors data has been investigated in this study. These sensors are generally employed for attitude estimation. The magnetometer and the horizon sensor measure the Earths magnetic field as well as the Earths center direction in the body frame, respectively. The magnitude of the magnetic field and the angle between two vectors have been used for orbit estimation purpose. This excludes the knowledge of the attitude in the orbit determination. The Gaussian variation of parameters equations is used for the orbital motion dynamical model to have the orbital elements as the states of the system. Since... 

Attitude control of a satellite having only two reaction wheels is a challenging issue. To address this problem, previously published researches considered some simplifying assumptions on the satellites such as diagonality of the moment of the inertia matrix. On the other hand, in some works, the total angular momentum of the satellite is assumed to be zero. In this paper, a linear time-variant model predictive control (LTV MPC) is designed to control a satellite with two reaction wheels. This control method can be applied to a satellite with a non-diagonal inertial matrix in the presence of external torques, to rotate the satellite toward the desired directions in the space and orbit. The... 

Accurate orbit determination (OD) is vital for every space mission. This paper proposes a novel heuristic filter based on adaptive sample-size Gaussian swarm optimization (AGSF). The proposed estimator considers the OD as a stochastic dynamic optimization problem that utilizes a swarm of particles in order to find the best estimation at every time step. One of the key contributions of this paper is the adaptation of the swarm size using a weighted variance approach. The proposed strategy is simulated for a low Earth orbit (LEO) OD problem utilizing geomagnetic field measurements at 700 km altitude. The performance of the proposed AGSF is verified using Monte Carlo simulation whose results... 

Purpose - The purpose of this paper is to propose a new guidance algorithm for launching a satellite using an expendable rocket from an equatorial site to an equatorial low-Earth orbit. Design/methodology/approach - Guidance during endoatmospheric portion is based on a nominal trajectory computed prior to take-off. A set of updating computations begins anew at the time instant tg of transition from endoatmosphere to exoatmosphere. The updating computations determine a guidance trajectory and an associated control law for the remainder of path by taking into account the rocket state at time tg. Thus, the overall guidance involves both initial and midcourse operations, and it has both open-... 

The weight of objects which are placed into low earth orbit should be closely managed historically a launch vehicles lift capability is fixed,so designers have paid close attention tothe weight of satellite subsystems to optimize the payload mass.Also due to applications of these structures,their performance against the vibrations is important.Simultaneous reductionof weight and band averaged vibration transmission in a satellite boom structure is the major task of the present study.This study approached the design problem by sizing periodic structureand geometric parameters to optimize the weight and system performance in a broadbandfrequency region using genetic algorithm (GA)  

The Earth albedo factor (EAF) is a major required parameter for the thermal analysis of low Earth orbit satellites. Satellites receive variable albedo radiation as they orbit around the Earth that is heavily dependent on the Earth's local terrain as well as the instantaneous cloud coverage. However, for satellite thermal balancing and control analysis, a constant mean EAF is usually taken based on the orbital parameters that could potentially introduce erroneous results. Recent advances in temperature-based orbit estimation (TBOE) algorithms have revealed a significant sensitivity concerning EAF giving rise to the idea of its on-line estimation for added accuracy. In this sense, a novel... 

The attitude fault-tolerant control of a flexible satellite actuated by reaction wheels and magnetic torquer bars is investigated in this article. A low earth orbit is considered for moment perturbations such as drag and gravity gradient. Furthermore, the flexible panels attached to a rigid central body are modeled through the assumed mode approach by a finite set of bending modal motion. The ordinary differential equations of their generalized coordinates are found using Lagrange’s equation, and the resulting dynamical model is validated by comparing its simulation results to the NX Siemens software results. Finally, a fault-tolerant controller based on sliding mode control is suggested and... 

In this paper, an autonomous orbit control of a satellite in Low Earth Orbit is investigated using model predictive control. The absolute orbit control problem is transformed to a relative orbit control problem in which the desired states of the reference orbit are the orbital elements of a virtual satellite which is not affected by undesirable perturbations. The relative motion is modeled by Gauss's variational equations including J2 and drag perturbations which are the dominant perturbations in Low Earth Orbit. The advantage of using Gauss's variational equations over the Cartesian formulations is that not only the linearization errors are much smaller, but also each orbital element can be... 

In this study, the optimal burn time for low-thrust impulsive propulsion systems is investigated to raise the perigee altitude of a low-Earth orbit. The maneuver is done using spin-stabilized attitude control and impulsive thrusting system for a time interval centered about apogee point. On the one hand, the low value of the thrust level causes more burn time needed to accomplish the transfer. This, in turn, will cause more thrust loss due to the deviation between the thrust axis (spin axis) and the velocity vector of the satellite. On the other hand, for small thrust duration, the transfer needs more revolutions around the Earth and more travel in lower altitudes with dense atmosphere and... 

Elevation angle is one of the most significant parameters of land mobile satellite channels, subject to rapid variations in the case of Low Earth Orbit (LEO) satellite systems. In this paper a novel trace-based framework is proposed and analyzed which is capable of predicting elevation angle as a function of time during satellite visibility window. Trace-time based modeling makes the framework practical for real-time evaluation of elevation angle and its alteration incurred by communication links in LEO satellite systems. The proposed method is particularly suitable for development of communication channel models and services in mobile LEO satellite networks where path variability is of... 

This paper presents a feasibility study on using satellite signal for passive radar application. Our focus is on utilizing geostationary Earth orbit satellite signal with suitable properties, like Inmarsat. Then, a new method of detection for passive coherent location using adaptive filter weights variation model is presented. To evaluate the performance, three different scenarios including a low Earth orbit satellite, a space shuttle, and a high-altitude aircraft as the targets of interest are considered. In addition to being covert, it will be shown that using such passive radar system, we can benefit the forward scatter enhancement, which enables us to detect such high-altitude targets... 

The Earth albedo factor (EAF) is a major required parameter for the thermal analysis of low Earth orbit (LEO) satellites. Satellites receive variable albedo radiation as they orbit around the Earth that is heavily dependent on the Earth's local terrain as well as the instantaneous cloud coverage. However, for satellite thermal balancing and control analysis, a constant mean EAF is usually taken based on the orbital parameters that could potentially introduce erroneous results. Recent advances in temperature-based orbit estimation (TBOE) algorithms have revealed a significant sensitivity concerning EAF giving rise to the idea of its on-line estimation for added accuracy. In this sense, a...